An electrical power delivery system is a complex system consisting of one or more generators with power flowing through cables to nodes, and then to loads. The functions required of the high-powered nodes are distribution, switching and power management. The functions of conversion and power conditioning are most appropriately handled at the branch level nodes. The node level functions are performed at high-power nodes in prior art legacy systems by circuit breakers and switch gear.
In the event of a fault, a prior art system may permit a high fault current, which has a potential for catastrophic collateral damage and which may also deprive other loads on the same or upwardly connected nodes of energy. When a fault occurs in the prior art system, a circuit breaker upstream from the fault opens. The prior art electromechanical circuit breaker may take up to 50 milliseconds to open for a high fault and 100 or more milliseconds for an intermediate fault. During these transient time periods, the systems upstream of the fault are perturbed. This perturbation is usually exhibited by a significant drop in voltage, particularly in close proximity to the fault, which may result in the voltage dropping to near zero for the period of time between the occurrence of the fault and the opening of the circuit breaker. This means that all loads being supplied by other circuits emanating from a node with a fault will experience a very low or zero voltage condition during the time of the fault. Sensitive loads may malfunction and some loads may become disconnected or may need to be reset or rebooted, causing them to be offline for a period of time significantly longer than the actual fault. This is obviously undesirable for sensitive and critical loads. Other loads may be transferred to alternate sources, which may cause further disturbances to the electrical system. In addition, there may be substantial arcing at the point of fault while the electromechanical circuit breaker is opening.
Such a scenario is shown in FIG. 1. In this example, there are 4 power panels (PP), each with six loads, fed from a load center node (LC). If a fault occurs at F1, with legacy equipment, the 18 loads in power panels #1, #2, and #3 will be deprived of power until the fault is cleared, which may take a minimum of 50 milliseconds and which could take as long as 400 milliseconds. The 6 loads in power panel #4 will be lost because the cable feeding them is faulted.
The parent to this application proposed a replacement for the electromechanical circuit breakers that currently detect and switch off faulted circuits which consisted of a device having two parallel current paths for each line (or phase). One path consisted of power electronic devices which could be gated to switch current on and off very quickly and the second, parallel path consisted of a mechanical contactor device which carries current very efficiently and which can open sufficiently quickly to commutate the current to the power electronic path in less than 25 microseconds. When a fault is detected, the mechanical contactor is tripped and the fault current is commuted to the power electronics path until the power electronics can be switched off. Using this configuration, it was possible to detect a high fault current within about 50 microseconds and to interrupt a high fault current in less than 400 microseconds. This innovation provided an approximate thousand-fold increase in speed over prior art legacy systems. In addition, it also was able to minimize or eliminate the arcing that traditionally occurs when an electromechanical circuit breaker is opened.
Once the fault current has been detected and commuted to the power electronics path, the flow of current from the source to the load can be interrupted by opening, or switching off, the power electronics path. The switch in the power electronics path typically consists of an IGBT which can be gated to interrupt the current flow.
One problem with this configuration is that the inductive energy stored in the source and load inductances must be dissipated in the interrupting switch in order to bring the circuit current to zero. The voltage that can be developed during interruption is the sum of the open circuit voltage of the source and the back EMF developed by the source and load inductances. As the interruption time decreases, dI/dt increases and the inductive voltage increases. As interruption time increases, the inductive voltage decreases, but the switch is forced to carry current while dropping the source voltage and so dissipates more energy. The switch can be destroyed either by excessive voltage or excessive dissipation (heating). There is an optimum opening time which limits voltage to a safe value, while dissipating the minimum energy.
In the current art, the switch is protected by employing a parallel snubber circuit. The role of the snubber circuit is to limit the voltage across the switch and absorb the energy from the circuit. Therefore, the switch can be opened as quickly as possible, while commutating current to the snubber circuit. The switch thereby dissipates minimum energy while the snubber circuit limits the voltage and absorbs the energy. The snubber circuit can be constructed with passive or active components or a combination of both.
One of the most common snubber circuits is the resistor-capacitor-diode (RCD) configuration in which a series resistor-capacitor with a diode across the resistor is attached in parallel with the switch. When the switch is opened, current flows through the diode into the capacitor, providing a low impedance path for the commutated current. The capacitor is sized such that the peak voltage, which is reached when the circuit energy has all been absorbed in the capacitor, is below the maximum allowable for the switch. When the switch is closed the diode then blocks voltage and forces the capacitor to discharge through the resistor. The resistor thus ends up dissipating the circuit energy. There are many variations on this approach which can include inductors, capacitors, resistors and diodes. One problem with this configuration, however, is that, in high power circuits, the size, weight and cost of these components is significant and therefore poses an important impediment to market acceptance.
An alternative approach to voltage and energy management is to use active components such as varistors with or without a series switch as the parallel snubber. A varistor is a nonlinear resistive element that displays high resistance at low voltage and low resistance above some threshold voltage. By selecting a varistor that has a threshold voltage above the circuit voltage, but below the safe limit of the switch, the voltage can be limited during rapid switch turn off, while the varistor is forced to absorb the circuit energy. Varistors do not have a sharp threshold voltage cutoff so adequate control of voltage sometimes requires selection of a low threshold voltage device which then leaks current and dissipates power during normal voltage operation. A series switch is then used to isolate the varistor during normal operation, and then connect it during interruption. Varistors are generally smaller than passive snubbers, but repeated operation deteriorates performance and the limited, and somewhat unpredictable, life of the device is a major impediment to broad application. The addition of a series switch improves life and reliability but with the penalty of another active component together with all the controls and auxiliaries necessary to operate it.
Therefore, it would be desirable to provide a circuit configuration which provides the same features as the snubber circuits of the prior art, but without the disadvantages and drawbacks associated therewith.